The present invention relates to a method for the production of an injection-moulded part as well as a device for the production of an injection-moulded part.
In particular when injection-moulding optical elements (for example lenses), it is known from the state of the art to inject-mould the injection-moulded parts to be produced in several layers or steps (for example from AT 505 321 A1). This has some advantages. For one thing, contour accuracy is improved by compensation of sink marks of a previously injection-moulded layer. Moreover, the pressure required to open the mould is reduced. Furthermore, the cycle time is shortened, because the cooling time increases, quadratically, with the wall thickness of the injection-moulded layer. This is shown, for example, in WO 2012/069590 A1. The sum of the cooling times in injection-moulding with several layers is thus less than in injection-moulding in one piece. This is shown in more detail below.
Generally, it can be assumed that the required cooling times are to be equal for all stations. This is advantageous especially if all the stations are arranged in a die or a machine.
The cooling time tk is proportional to the square of the wall thickness, thus tk=A·s2 applies when using a factor of proportionality A.
In contrast to the inner layer, produced first, in the die outer layers are cooled only on one side. The cooling time of a layer cooled on one side is approximately equal to the cooling time of a layer cooled on both sides that is twice as thick. In order to achieve equal cooling times, an outer layer is thus permitted to have only half the wall thickness of the inner layer. Therefore, for an injection-moulded part made of three layers with the overall wall thickness s, in a first approximation a layer thickness distribution of s2=¼s, s1=½s, s3=¼s is useful. (s2, s3 etc. denote the layer thicknesses of the outer layers, s1 that of the inner layer. s denotes an overall thickness.)
Analogously, with an injection-moulded part of 2n+1 layers, the thickness of the first (inner) layer can be assumed to be
      1          n      +      1        ,and the thickness of all subsequent layers
      1          2      ·              (                  n          +          1                )              ·  nis a natural number greater than or equal to 1, which indicates how often a pre-moulded part is subsequently overmoulded on both sides. The sum of the cooling times of the individual stations can be used to assess the cycle time saving. Although the individual cycle times can run in parallel, every station requires space in the die and in a machine—in conventional single-layer technology this space could have been used to accommodate further cavities.
Assuming that due to the cooling on one side the cooling time of the outer layers s2 and s3 corresponds to the cooling time of a layer that is twice as thick, the sum of the cooling times in a three-layer injection-moulded part is now
            t              k        ,        total              =                            t                      k            ⁢                                                  ⁢            1                          +                  t                      k            ⁢                                                  ⁢            2                              =                                    A            ·                                          (                                  s                  2                                )                            2                                +                      A            ·                                          (                                                      2                    ⁢                    s                                    4                                )                            2                                      =                              2            ⁢                          A              ·                                                (                                      s                    2                                    )                                2                                              =                                    1              2                        ⁢                          A              ·                              s                2                                                          ,wherein the cooling time of the layers s2 and s3 must be taken into account only once, as the cooling takes place at the same time and in the same station.
It is to be recognized that the total cooling time for the individual layers is only half the cooling time tk=A·s2 of a single-layer injection-moulded part.
The following applies for n layers:
      t          k      ,      total        =            1              n        +        1              ·    A    ·          s      2      
The total cooling time is now only the fraction 1/(n+1) of the cooling time of a single-layer injection-moulded part.
In addition to the obvious improvement in productivity, this reduced cycle time also has, for example, the advantage that the residence time of the material in the space in front of the screw is reduced.
In spite of this reduction in the cooling time, this still makes up a considerable proportion of the overall cycle time.